The present invention relates to an electrical-discharge wire cutting machine, and more particularly to a method of quickly and accurately positioning wire guides so that a wire electrode guided by the wire guides will be directed perpendicularly to a plane in which a workpiece moves, and a method of obtaining data on the exact span or distance between the wire guides which is required for subsequent tapered surface machining.
Electrical-discharge wire cutting requires a wire electrode to be directed perpendicularly to a reference machining plane, i.e., a plane in which a workpiece moves. The perpendicularity of the wire electrode to the reference machining plane is highly important since it directly affects the accuracy of machining. Another important element required by electrical-discharge wire cutting is the accurate measurement of a span or distance between upper and lower wire guides by which the wire electrode is guided. The data on the accurately measured wire guide span are required for accurate tapered surface machining.
There are known two typical methods of directing a wire electrode perpendicularly to a reference machining plane. One method is disclosed in Japanese Patent Application (Kokai) No. 54(1979)-104099. The disclosed method is carried out with respect to an electrical-discharge wire cutting machine which includes a workpiece table movable along an X-axis and a Y-axis that extend perpendicularly to each other, and a moving unit for moving one of upper and lower wire guides parallel to the X-axis and the Y-axis. On the workpiece table, there is mounted a wire electrode perpendicularity gage having first and second detecting surfaces. The first detecting surface is directed normal to the X-axis and is composed of upper and lower detectors. The second detecting surface is directed normal to the Y-axis and is composed of upper and lower detectors. In operation, the workpiece table is moved in one direction along the X-axis until one of the upper and lower detectors of the first detecting surface contacts a wire electrode. When the contact between one of the detectors and the wire electrode is detected, said one of the upper and lower wire guides is moved by a predetermined small distance parallel to the X-axis in a direction away from the detector which was contacted by the wire electrode or in a direction toward the detector which was not contacted by the wire electrode, depending on whether the wire electrode contacted the upper or lower detector. The above process is repeatedly carried out until the upper and lower detectors simultaneously contact the wire electrode. Then, the same operation as above is effected with respect to the second detecting surface and the Y-axis, thereby holding the wire electrode perpendicularly to the reference machining plane.
The other method is disclosed in Japanese Patent Application (Kokai) No.1(1989)-103229. This method is carried out with respect to the same electrical-discharge wire cutting machine as described above. One of the wire guides is moved parallel to the X-axis in order to tilt the wire electrode. The workpiece table is then moved in one direction along the X-axis until one of the upper and lower detectors of the wire electrode perpendicular gage contacts the wire electrode. Thereafter, the workpiece table is moved by a distance (d) in a direction to displace the wire electrode perpendicularity gage away from the wire electrode, and then said one wire guide is moved by a distance (e) parallel to the X-axis in a direction to displace the wire electrode guided by that wire guide further away from the wire electrode perpendicularity gage. Next, the workpiece table is moved by a distance (f) in one direction parallel to the X-axis until the other detector of the wire electrode perpendicularity gage contacts the wire electrode. Based on the above moving distances d, e and f, values inherent in the machine which have been inputted as distances a and b between the wire guides and detectors close thereto, and a previously measured distance W between the upper and lower detectors, computed is the distance required for the first-moved wire guide to move for bringing the wire electrode perpendicularly to the reference machining plane. (The terms used in the above description of the prior art are not in conformity with the terminology used in the above publications, but are the same as those used in the present invention for easier comparison between the prior art and the present invention. However, the symbols remain the same as those found in the publications.)
According to the former method, the wire guides are repeatedly moved by small distances until the upper and lower detectors simultaneously contact the wire electrode. Therefore, a long period of time is needed to bring the wire electrode perpendicularly to the reference machining plane, and hence low working efficiency and low productivity result.
According to the latter method, the distance required for the first-moved wire guide to move for directing the wire electrode perpendicularly to the reference machining plane is computed using the previously measured distance W between the upper and lower detectors and the inherent values which have been input as the distances a, b between the wire guides and detectors close thereto.
In ordinary electrical-discharge wire cutting machines, the upper and lower wire guides are consumed very rapidly, and need to be replaced each time the machine has operated over a certain period of time. When the wire guides are replaced, the distances a, b slightly vary because of shape and dimensional irregularities of the wire guides. Therefore, the above method which uses the inherent values which have been inputted as the distances a, b cannot direct the wire electrode exactly perpendicularly to the reference machining plane when the wire guides are replaced. Further, this conventional process which has been proposed to measure the accurate values a, b is self-contradictory and is not practical, since these values are measured after the entire method of directing the wire electrode perpendicularly to the reference machining surface is presumed to be completed.